In a device having an analogue signal path, for example radio transmitters, radio receivers, ultrasonic receivers and other applications, the gain of that signal path is usually adjustable, commonly over a wide range. Traditionally, an analogue control voltage or current has been used to vary the gain using a voltage or current controlled amplifier. However, it is becoming increasingly desirable to adjust the gain of a system using stages that can have their gain switched digitally, since digitally switched gain control stages potentially have improved power consumption or generate less electrical noise. In a completely digital system the gain control signal provided to the digitally switched gain control stages can itself be a digital signal. However, it is also desirable to retain the facility to accept an analogue control input signal to a digitally switched gain control stage for reasons of compatibility or interchangeability with existing devices, or for use inside an analogue feedback loop, amongst other reasons.
One possible approach is to simply digitize the analogue gain control signal using an analogue to digital converter (ADC) and then use the resulting digital value to control the digitally controlled gain stages, together with any present analogue controlled gain stages via a digital to analogue converter (DAC). However, a disadvantage with this approach is that there are gain values in between the possible output values of the ADC that are impossible to obtain, because of the finite resolution of the ADC. Additionally, in applications such as some mobile telephone transmitters, where the gain control voltage may change only infrequently, but where the gain of the signal chain must respond rapidly when the gain control signal does change, there is an additional problem with the use of an ADC to digitize the control voltage. In some systems no information is available to describe at which point in time a change in the gain control signal will take place. If it is not known when the gain control voltage is liable to change, then to respond quickly to changes in the gain control voltage, the ADC must be continuously converting the analogue input signal, which will mean that there is a continuous power consumption in the ADC, which is generally undesirable. Furthermore, there will therefore be continuous switching activity in the ADC, which is liable to generate spurious radio frequency signals, which are also undesirable.